Noninvasive reflectance pulse oximetry has recently become an important new clinical technique with potential benefits in fetal and neonatal monitoring. The main reason for this application is the need to measure the relative concentration of oxygenated hemoglobin in arterial blood, SaO2, from multiple convenient locations on the body (e.g. the head, torso, or upper limbs), where conventional transmission pulse oximetry cannot be used. Using reflectance oximetry to monitor SaO2 in the fetus during labor, where the only accessible location is the fetal cheek or scalp, provides additional convenient locations for sensor attachment.
While transmission and reflection pulse oximetry are based on similar spectrophotometric principles, it is widely known that reflection pulse oximetry is more challenging to perform and has unique problems. Reflection pulse oximetry can be adversely affected by strong ambient light generated for instance by light sources in the operating room or other light sources used for patient examination or phototherapeutic interventions. Another practical problem in reflection pulse oximetry is the generally very weak pulsatile AC signals that are typically about 10 to 20 times smaller in amplitude compared to AC signals detected by transmission mode pulse oximeter sensors. Consequently, the normalized AC/DC ratios derived from the reflected R or IR photoplethysmograms that are used to compute arterial oxyhemoglobin saturation, SpO2, are very small and range from about 0.001 to 0.005 depending on sensor configuration or placement. In addition, the small amplitudes add considerable noise often leading to unstable readings, false alarms and inaccurate measurements of SpO2.
Improving the quality of the detected photoplethysmographic signals in reflectance pulse oximetry will be beneficial, since inaccuracies caused by noisy and weak pulsatile signals remain one of the major unsolved sources of errors in reflectance pulse oximetry.